ATP and NADPH as the driving force of carbon reduction in leaves in relation to thylakoid energization by light

Carbon assimilation of spinach (Spinacia oleracea L.) leaves was measured in the presence of 2000μl· l−1CO2 and 2% O2 in the gas phase to suppress photorespiratory reactions and to reduce stomatal diffusion resistance. Simultaneously, membrane parameters such as modulated chlorophyll fluorescence, oxidation of P700 in the reaction centre of photosystem I, and apparent changes in absorbance at 535 nm were recorded. After light-regulated enzymes were activated at a high irradiance, illumination was changed. About 3 min later (to maintain the previous activation state of enzymes), leaves were shock-frozen and freeze-dried. Chloroplasts were isolated nonaqueously and analysed for ATP, ADP, inorganic phosphate, NADPH and NADP. Observations made under the chosen conditions differed in some important aspects from those commonly observed when leaves are illuminated in air. (i) Not only assimilation, but also the phosphorylation potential [ATP]/([ADP]·[Pi]) increased hyperbolically with irradiance towards saturation. In contrast, the ratio of NADPH to NADP did not change much as irradiances increased from low to high photon flux densities. When ATP, the phosphorylation potential and the assimilatory force, FA (the product of phosphorylation potential and NADPH/NADP ratio), were plotted against assimilation, ATP increased relatively less than assimilation, whereas the phosphorylation potential increased somewhat more steeply than assimilation did. A linear relationship existed between assimilation and FA at lower irradiances. The assimilatory force FA increased more than assimilation did when irradiances were very high. Differences from previous observations, where FA was under some conditions higher at low than at high rates of carbon assimilation, are explained by differences in flux resistances caused not only by stomatal diffusion resistance but also by differences in the activity of light-regulated enzymes, (ii) The relationship between P700 oxidation and a fast absorption change with a maximum close to 520 nm on one hand and carbon assimilation on the other hand was largely linear under the specific conditions of the experiments. A similar linear relationship existed also between the quantum efficiency of electron flow through photosystem II and the quantum efficiency of photosystem I electron transport. (iii) Whereas the increase in non-photochemical fluorescence quenching, qN, was similar to the increase in assimilation, the relationship between light scattering and assimilation was distinctly sigmoidal. Light scattering appeared to be a better indicator of control of photosystem II activity under excessive irradiation than qN. (iv) The results are discussed in relation to the relative significance of chloroplast levels of ATP and NADPH and of the assimilatory force FA in driving carbon assimilation. From the observations, the proton/electron (H+/e−) ratio of linear electron transport is suggested to be 3 and the H+/ATP ratio to be 4 in leaves. An H+/e− ratio of 3 implies the existence of an obligatory Q-cycle in leaves.

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